The present invention generally relates to magnetic heads and more particularly to an improvement of a GMR (giant magneto-resistance) head having a so-called spin-valve structure.
A GMR head is a high-sensitivity magnetic head that detects a change of resistance of a magnetic layer that in turn occurs in response to a change of direction of a very weak external magnetic field. Because of the high magnetic sensitivity, a GMR head is expected to play a major role in a high-density magnetic recording apparatus as a high-resolution and high-sensitivity magnetic head.
FIG. 1 shows the overall construction of a magnetic head 10 having a typical conventional spin-valve structure, while FIG. 2 shows the construction of a spin-valve head 14 used in the magnetic head of FIG. 1.
Referring to FIG. 1, the magnetic head 10 includes a lower magnetic shield layer 12 of a magnetic material such as FeNi, CoFe or FeN provided on a substrate 11 of Al.sub.2 TiC. On the foregoing lower magnetic shield layer 12, there is provided a spacer layer 13 of a non-magnetic material such as A1.sub.2 O.sub.3, and a magnetic sensor 14 having a spin-valve structure is formed on the spacer layer 13.
The magnetic sensor 14 is covered by another spacer layer 15 also of a non-magnetic material similar to the spacer layer 13, and an upper magnetic shield layer 16 of a soft magnetic material such as FeNi or CoFe is provided on the spacer layer 15. Thereby, the spacer layer 13, the magnetic sensor 14 and the spacer layer 15 form together a minute magnetic read gap between the upper and lower magnetic shield layers 12 and 16 with a size of about 200 nm.
On the upper magnetic shield layer 16, there is provided another spacer layer 17 of a non-magnetic material with a thickness of about 350 nm, and a coil pattern 19 is provided on the spacer layer 17 with an intervening insulation layer 18, wherein the insulation layer 18 continuously has a reducing thickness thereof toward a front end 10A of the magnetic head 10. The coil pattern 19 is covered by another insulation layer 20, and a magnetic pole 21 of a magnetic material such as FeNi or CoFe is provided on the foregoing another insulation layer 20 such that the thickness of the insulation layer 20 decreases continuously toward the foregoing front end 10A of the magnetic head 10. As a result of the decreasing thickness of the insulation layers 18 and 20 at the front end 10A of the magnetic head 10, the magnetic pole 21 makes direct contact with the spacer layer 17 at the front end 10A. There, a minute magnetic write gap is formed between the upper magnetic shield 16 and the magnetic pole 21. It should be noted that the upper magnetic shield 16 extends to the magnetic pole 21 at a part not illustrated in FIG. 1, and a magnetic circuit is formed.
The magnetic head 10 scans the surface of a magnetic recording medium such as a magnetic disk at the foregoing front edge surface 10A, and the magnetic sensor 14 detects the magnetization recorded on the surface of the magnetic recording medium at the foregoing magnetic read gap. Further, a recording of information is made on the magnetic recording medium at the foregoing write gap by energizing the coil 19 by an information signal.
FIG. 2 shows the construction of the magnetic sensor 14 in detail, wherein those parts explained already with reference to FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 2, the magnetic sensor 14 includes a magnetic detection layer or so-called "free layer" 14A of a soft magnetic material such as CoFe or NiFe formed on the spacer layer 14, wherein the free layer changes the direction of magnetization freely in response to the magnetization of the magnetic recording medium.
On the free layer 14A, there is provided an intermediate layer 14B of a non-magnetic material such as Cu, and a fixed magnetization layer or so-called "pinned layer" 14C is provided on the intermediate layer 14B with a predetermined fixed magnetization, wherein the pinned layer 14C is formed of a soft magnetic material such as CoFe or NiFe similarly to the free layer 14B. It should be noted that the magnetization of the pinned layer 14C is fixed in the direction of magnetization of a magnetization-fixing layer or so-called "pinning layer" 14D, wherein the pinning layer 14D is formed of an anti-ferromagnetic material such as FeMn or PdPtMn and provided on the pinned layer 14C. It should be noted that the pinning layer 14D fixes the magnetization of the pinned layer 14C by spin-exchange interaction. Thereby, the magnetization of the magnetic recording medium is detected by detecting a change of electric resistance that occurs in response to the change of direction of magnetization in the free layer 14A with respect to the direction of magnetization in the pinned layer 14c. In FIGS. 1 and 2, it should be noted that the electrodes for detecting the foregoing resistance change is omitted from illustration. Further, it should be noted that the pinning layer 14D, lacking a spontaneous magnetization, is relatively immune to the external magnetic field.
In the magnetic sensor 14 having such a construction, in which the direction of magnetization of the free layer 14A changes in response to the direction of magnetization of the magnetic recording medium, it should be noted that the resistance of the magnetic sensor 14 becomes minimum when the direction of magnetization of the layer 14A is parallel to the direction of the magnetization of the pinned layer 14C. When the direction of magnetization of the layer 14A is in an anti-parallel relationship with the direction of magnetization of the pinned layer 14C, on the other hand, the resistance of the magnetic sensor 14 becomes maximum.
In the case of using the magnetic sensor 14 for the magnetic head 10, it is advantageous to set the direction of magnetization of the pinned layer 14C perpendicular to the direction of magnetization of the free layer 14A in a free state in which there is no external magnetic field applied to the magnetic sensor 14. See FIG. 3A. By doing so, the resistance of the magnetic sensor 14 is increased or decreased generally symmetrically depending on whether the magnetization of the magnetic recording medium is parallel or anti-parallel to the magnetization of the pinned layer 14C, as indicated in FIG. 3B. Such a generally symmetric increase and decrease of the resistance facilitates the signal processing in the magnetic recording and reproducing apparatus.
It should be noted that the control of magnetization of a magnetic body is conducted in a heat treatment process.
FIG. 4 shows a heat treatment process conducted conventionally in the process of forming the spin-valve structure of FIG. 2.
Referring to FIG. 4, the spin valve structure of FIG. 2 is formed in a step 1 of FIG. 4 by depositing the layers 14A-14D under the existence of an initial magnetic field with a predetermined initial direction designated as a 0.degree. direction. The free layer 14A thus formed has an easy axis of magnetization in the foregoing 0.degree. direction.
Next, in the step 2 of FIG. 4, the spin-valve structure of FIG. 2 is subjected to a thermal annealing process to a temperature close to a blocking temperature of the pinning layer 14D, and a magnetic field is applied in the foregoing 0.degree. direction as indicated by blank arrows. As a result, the direction of magnetization of the pinned layer 14C is aligned in the 0.degree. direction.
FIGS. 5A and 5B explain the blocking temperature.
In a magnetic system in which an anti-ferromagnetic layer and a soft magnetic layer form an exchange coupling as in the case of the spin-valve structure of FIG. 2, it should be noted that the hysteresis curve of the magnetic system is displaced along the horizontal axis representing the magnetic field H by an amount Hua as indicated in FIG. 5A as a result of the pinning of magnetization caused in the soft magnetic layer such as the pinned layer 14C by the anti-ferromagnetic pinning layer 14D. This phenomenon means that it is insufficient in a magnetic system that includes such an anti-ferromagnetic layer, to apply a magnetic field just enough to cause an inversion of magnetization in an ordinary magnetic system not including an anti-ferromagnetic layer, for causing an inversion of magnetization. In order to achieve this, it is necessary to increase the magnetization by an amount Hua. This is the pinning of magnetization.
FIG. 5B shows the temperature dependence of the quantity Hua.
Referring to FIG. 5B, it should be noted that the quantity Hua decreases with increasing temperature and reaches zero at a blocking temperature T.sub.B. In other words, the magnetization of the spin valve structure of FIG. 2 can be controlled as desired by an external magnetic field when the system is heated to a temperature close to or higher than the blocking temperature T.sub.B. The pinning of magnetization in such a magnetic system is eliminated when the system is heated to the blocking temperature T.sub.B or higher.
Referring back to FIG. 4, the pinned layer 14C is magnetized in the foregoing 0.degree. direction as a result of the thermal annealing process conducted at the temperature close to the blocking temperature T.sub.B, and the anti-ferromagnetic pinning layer 14D forms an exchange coupling with the pinned layer 14C. By cooling the structure thus formed to room temperature environment in the step 3 of FIG. 4, a structure in which the pinned layer 14C is magnetized in the 0.degree. direction is obtained. In the structure of step 3 of FIG. 4, the magnetization of the pinned layer 14C is pinned by the pinning layer 14D. As a result of the thermal annealing process in the step 2 of FIG. 4, the magnetic shield layers 12 and 16 and the magnetic pole 21 forming the magnetic head 10 of FIG. 1 are all magnetized in the foregoing 0.degree. direction.
Next, in the step 4 of FIG. 4, the structure of step 3 of FIG. 4 is heated to a temperature close to the blocking temperature T.sub.B again, and an external magnetic field is applied to the spin valve structure in a direction perpendicular to the foregoing 0.degree. direction. As a result, the magnetization of the pinned layer 14C is rotated by 90.degree., and a structure shown in step 5 of FIG. 4 is obtained, in which it should be noted that the direction of magnetization of the pinned layer 14C is perpendicular to the direction of easy axis of magnetization of the free layer 14A.
In the process of FIG. 4, however, there arises a problem, associated with the fact that the entire magnetic head 10 including the spin-valve magnetic sensor 14 is heated to the temperature close to the blocking temperature T.sub.B, in that the direction of magnetization of the magnetic shields 12 and 16 or the direction of magnetization of the magnetic pole 21, which has been initialized in the step 2, may be changed unwantedly. Further, such thermal annealing and magnetization processes conducted at the temperature close to the blocking temperature may cause a 90.degree. rotation in the easy axis of magnetization from the desired 0.degree. direction as indicated by a broken arrow in step 5 of FIG. 4. When this occurs, the detection characteristic of the magnetic sensor 14 is modified from the characteristic of FIG. 3B and the magnetic sensor 14 would produce an asymmetric output in response to the magnetization of the magnetic recording medium.